The Response to DNA Damage at Telomeric Repeats and Its Consequences for Telomere Function

Abstract

Telomeric repeats, coated by the shelterin complex, prevent inappropriate activation of the DNA damage response at the ends of linear chromosomes. Shelterin has evolved distinct solutions to protect telomeres from different aspects of the DNA damage response. These solutions include formation of t-loops, which can sequester the chromosome terminus from DNA-end sensors and inhibition of key steps in the DNA damage response. While blocking the DNA damage response at chromosome ends, telomeres make wide use of many of its players to deal with exogenous damage and replication stress. This review focuses on the interplay between the end-protection functions and the response to DNA damage occurring inside the telomeric repeats, as well as on the consequences that telomere damage has on telomere structure and function.

Keywords: alternative lengthening of telomeres; end-protection problem; shelterin complex; telomere damage; telomere maintenance; telomere replication; telomeric double strand breaks.

Figure 1

Telomere structure and the shelterin complex. (A) Illustration of the shelterin complex loaded on the telomeric DNA. Shelterin components TRF1 and TRF2 are shown as dimers. For the sake of simplicity only one complex is shown, although many complexes bind throughout the telomeric repeats. (B) Illustration of the telomere in the t-loop configuration, where the 3′ telomeric overhang has invaded the internal repeats, pairing with the complementary C-rich strand. The actual structure at the base of the t-loop is not known and multiple configurations are possible [7].

Figure 2

End-protection and the response to telomeric Double Strand Breaks (DSBs). This model summarizes the data on the control of the different aspects of the DSB response throughout the telomeric repeats. Active pathways are shown in green, whereas repressed ones are shown in red. The chromosome terminus is shown in a t-loop configuration. Although the t-loop structure could provide protection from all the pathways indicated, redundant repression mechanisms exist for c- and alt-Non-Homologous End Joining (NHEJ) pathways at chromosome ends. See text for details.

Figure 3

Repair of telomeric DSBs through alt-NHEJ. Illustration of alt-NHEJ repair of a telomeric DSB. Processing of telomeric DSBs generates perfectly cohesive ends. Telomeric repeats are shown in two colors for visual purposes. Note that even if the two broken ends become dissociated, homology-guided repair will always promote correct rejoining of a broken telomere piece over generation of telomere fusions.

Figure 4

Telomere loss due to replication problems. (A) Illustration of the terminal fork entering the telomeric repeats. Possible structural transitions like fork reversion and processing are not shown. In the event of nucleolytic cleavage of the telomeric fork, the distal part of the sister telomere will not be replicated, resulting in abrupt telomere truncation. (B) Potential obstacles that the replication fork can encounter on the telomeric repeats. The G-rich telomeric repeats have a tendency to form G-quartets (G4) that can interfere with replication. The abundance of Guanines makes telomeres more susceptible to oxidative stress (8-oxo-G) compared to other genomic regions. Telomeres are continuously transcribed by PolII to generate the Telomeric-Repeat-Containing RNA (TERRA), a process that can potentially interfere with replication fork progression. Other potential obstacles to the replication fork are the presence of the tightly-bound shelterin complex (for simplicity, only one complex is shown in the figure) and the t-loop structure.

Figure 5

Possible mechanisms of DSB-induced telomere elongation in ALT cells. Telomeric DSB can initiate long-range telomere synthesis. The G-rich strand is represented in purple and the C-rich strand in light blue. Note that for telomere elongation to occur, the 3′ G-rich overhang of the telomere proximal DSB end needs to be engaged in strand invasion. Left: strand invasion of the broken end into a donor telomere will lead to a Break-Induced Replication (BIR)-mediated telomere elongation. A migrating bubble (d-loop) will copy telomeric repeats from the strand invasion point towards the end of the donor telomere. The C-rich strand could then be synthesized through asynchronous lagging strand synthesis, which might involve the use of specialized factors at telomeres, like the CST complex, normally involved in the fill-in of the 3′ overhang [41]. It is not clear what happens when a BIR intermediate encounters the t-loop structure; it is possible that the RTEL1 helicase, which has been implicated in t-loop resolution during replication, plays a similar role in this context. Right: telomeric DSBs could engage in strand invasion/annealing with Extra Chromosomal Telomeric Circles (ECTC) that are abundant in Alternative Lengthening of Telomeres (ALT) cells and often contain single-stranded gaps (C-circles). Strand invasion could proceed in rolling circle amplification that can engage in long-range telomere synthesis while displacing the damaged strand of the C-circle. In both BIR and rolling circle amplification models, telomere elongation occurs in a conservative manner, with both strands being newly synthesized.